122 PRIMITIVE FORMS OF LIFE 



In order to succeed in remaining without too much effort between 

 two layers of water, it was obliged to possess the same weight as 

 the water it displaced ; and its constituent substances had to be 

 carefully distributed so as to give it this quality, as some of 

 them were heavier than water, and others, such as the fats, were 

 lighter. But this could only happen in exceptional cases. If 

 it were lighter than water, it would be drawn towards the 

 surface and exposed to all sorts of accidents. Hence the heavier 

 forms which were naturally drawn towards the bottom of the 

 sea had the richest potentialities for the future. Under these 

 conditions the merid continued to elongate in the direction of 

 its trajectory. The end which went first and which had to 

 explore the ground to which the rest of the body had to be com- 

 mitted, became differentiated from the posterior. Its constituent 

 parts acquired a greater and greater sensibility, and a fair 

 number were transformed into nervous elements, distributed 

 along the exploratory tentacles or grouped together to form 

 eyes. A little behind the latter came the mouth, naturally 

 preceded by the exploratory region. The animal, probably 

 by virtue of a tactile sense, without which it would have been 

 unable to live, forced this mouth toward the ground on which 

 it crawled in its search for food. Thus, the mouth became a 

 feature of the ventral side, which was, moreover, flattened 

 by its own pressure against the ground, light as this must have 

 been. As the ventral side and the whole periphery of the mouth 

 were thus constantly stimulated by contact with the earth, the 

 development of numerous sensitive cells within this area 

 finally produced around the latter a ring of nerve tissue, and 

 on the ventral side a similar strand. Little by little — in 

 certain embryos, indeed, all the stages of this phenomenon 

 can be followed — the nerve cells became isolated from the 

 epidermis of which they had at first formed part, and 

 eventually came to constitute in the adult creature, the 

 pharyngeal collar, and the nerve-chain, which is found in a more 

 or less modified form in all Arthropods, Annelid Worms, 

 Echinoderms, and Molluscs. 



Locomotion has had even more influence on the final 

 evolution of the mobile protomerids than on their forms. The 

 same reason that determined the evolution of the fixed merids by 

 budding, holds good throughout for the free merids, except that 

 in the latter the buds would not be arranged in the same way. 



